How to "Chamfer" or Bevel Outer Edges of a Plane like in 3DS Max?
On an edge selection,CtrlShiftR, Offset Edge Slide, will get you most of the way there. It will slide a copy of selected edges along existing ones, and can be set to give an even offset. However, it doesn't create rail edges (in your case, at the corners) where they did not previously exist .. you would have to go round and J connect the vertices at the corners.

If you had to do so many of those it would be tedious, you could:


Extrude your selected edges any old where, so they are no longer on a boundary
Select the top faces I and inset the whole region with 'Boundary' unchecked
Alt-select the extruded face ring, and delete it.


Or, you could go round with K cutting across existing edges, making a  rough inset, and create the corner-rails, with no concern for accuracy, and then use GG to slide it, hitting E to make the edges parallel to the boundary. (F flips which side of the slide to be parallel to.)